An anthracene appended β-cyclodextrin based dyad: Study of self – assembly and photoinduced electron transfer processes

Abstract

The self-assembly of a beta cyclodextrin (beta-CD) based supramolecular dyad is reported, in which the donor anthracene moiety is covalently linked to the smaller rim of the beta-CD and the acceptor pyromellitic diimide (PMDI) is encapsulated within the beta-CD cavity. Encapsulation of the PMDI into beta-CD cavity was studied by a variety of techniques, which suggested that PMDI is encapsulated so as to position the aromatic part at the centre of the cavity with the 2-propyl end at the narrower rim among the overhanging primary OH groups and the N-ethylpyridinium end situated at the wider rim exposed to water. Photoinduced electron transfer (PET) in the system was studied by fluorescence quenching and laser flash photolysis techniques. At [PMDI]<10 -4M, the equilibrium is in favour of the free molecules and under these conditions fluorescence quenching is negligible and diffusion mediated electron transfer involving the triplet excite state of anthracene predominates. At higher concentrations of PMDI the equilibrium is largely in favour of the supramolecular dyad and intra-ensemble PET processes predominate. The experimentally determined electron-transfer rate constant agrees very well with that calculated by using the Marcus equation. It was observed that a fraction of the ion pairs survived for more than 200 Mus.